cmd/ld: don't delete output binary if not "ordinary" file.
cmd/ld: don't delete output binary if not "ordinary" file.

e.g., don't delete /dev/null. this fix inspired by gnu libiberty,
unlink-if-ordinary.c.

Fixes issue 7563

LGTM=iant
R=golang-codereviews, iant, 0intro
CC=golang-codereviews, r
https://codereview.appspot.com/76810045

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old